Compare to the traditional limit equilibrium methods, finite element method has some obvious advantages for analyzing slope stability. As a result, strength reduction factor method based on the finite element theory is widespread used. The paper presents the fundamental principle of the shear strength reduction approach for finite element method calculations of slope factor of safety. Also, the influence factors for accuracy of the slope safety factor were discussed. It is found that slope model should be built by the sixnode triangle type, and the element size ratio is 0.03, and it is found that distance from the crest of the slope to the left boundary should be more than 2.5 times of the slope width; the distance from the toe of the slope to the right boundary should be at least 2 times of the slope width; and the distance from the toe of the slope to the bottom boundary should be at least 2 times of the slope height. Finally, based on the optimal initial slope model, the safety factor of the initial end-wall slope in the studied coal mine is 1.52. The outcome shows that the simulation results by optimal slope model have the good accuracy.